The present invention relates to an oxide film which is suitable for a heat ray cut-off film for buildings or automobiles or which is suitable for a transparent conductive film for transparent electrodes, a laminate employing such oxide films, and methods for their production.
An oxide film comprising an oxide of a metal such as In, Zn or Sn as the main component, is widely used not only in the field of transparent conductive film materials but also in various other fields.
As a method for forming such an oxide film on a glass or transparent plastic substrate, a vacuum vapor deposition method, an ion plating method or a sputtering process is used. Among them, the sputtering process is a process whereby coating can be carried out at a high productivity uniformly over a large area, and it is suitable for forming a film over a large area, such as a touch panel or a liquid display element for a flat panel display.
For example, in a case where an ITO (a double oxide of indium and tin) film is to be formed by a sputtering process, there is 1) a method wherein a mixed sintered body of indium (In) oxide and tin (Sn) oxide is used as a target, and this target is subjected to sputtering, or 2) a method by so-called reactive sputtering wherein an alloy target of In and Sn is subjected to sputtering in an oxidizing atmosphere of a gas mixture of Ar and O2, and film forming is carried out while oxidizing metal atoms ejected from the target.
It is well known that electrical conductivity of ITO is substantially influenced by oxygen deficiency as well as by whether or not Sn oxide doped to In oxide as the main component will effectively work as a dopant. Accordingly, control of the degree of oxidation of the film is important.
In the former method wherein a mixed sintered body of In oxide and Sn oxide is used as a target, if pure Ar is used as the sputtering gas, it is usually possible to obtain only a film showing a brown color absorption due to oxygen deficiency. In order to supplement the oxygen deficiency, it is common to carry out sputtering by adding O2 a little as an oxidizing gas to the sputtering gas. However, if O2 is still inadequate, a brown color-absorbing film will be obtained, and if O2 is too much, the resistivity of the film will abruptly increase. Namely, in order to obtain a film having uniform properties with good reproducibility, fine control of the amount of O2 incorporated is essential. Thus, due to a change with time of the pumping speed of a vacuum pump or due to a change in the O2 concentration in a very small degree attributable to a change with time of the gas supply system, it has been likely that the oxidation is excessive, thus leading to a high resistivity film, or the oxidation is inadequate, thus leading to an absorbing film, and it has been difficult to obtain a film having the same properties with good reproducibility.
Further, in a case where a film is to be formed over a large area of a large-sized substrate, if a distribution occurs in the O2 concentration in the chamber, in plane irregularity in the properties will result, and it has been difficult to obtain a uniform film. Thus, application of a transparent conductive film has been limited to a substrate having a small surface area.
In a conventional method, it has been almost impossible to control the resistivity of a film by adjusting the O2 amount, since the resistivity increases abruptly if the O2 amount is increased in order to obtain a high resistivity film.
The latter reactive sputtering process is substantially inexpensive as compared with the alloy target or a sintered target and thus has a substantial merit from the viewpoint of costs. On the other hand, the oxidation degree of a film which is substantially influential over the electrical conductivity, is controlled by the flow rate ratio of the O2 gas in the sputtering with a gas mixture of Ar and O2. By this control, the change from the oxidation deficient state (the state to form a film showing optical absorption) to the excessive oxidation state (the state to form a film having high resistivity) is abrupt, and the conditional range has been very narrow for the intermediate region (the region where a transparent film having low resistivity can be obtained). Further, due to an influence of the partial pressure of water as the residual gas component, it has been difficult to obtain the optimum condition constantly or with good reproducibility. Further, there has been a problem that due to a distribution or change of the condition within the apparatus, an in-plane distribution is likely to result in the resistance, the film thickness or the optical properties such as transmittance and haze, and this tendency is strong especially when a film is formed on a substrate having a large surface area. Thus, by the prior art, control of the oxidation degree to attain electrical conductivity has not been adequate, and it has not been easy to constantly obtain the condition for good electrical conductivity. Further, for application to a substrate having a large surface area, an improvement in the in-plane uniformity of electrical properties and optical properties has been desired.
Further, if it is attempted to obtain an electrical conductive thin film having high resistivity by controlling the resistance, the resistivity is likely to change excessively due to the oxidation state, and it has been difficult to attain the desired high resistance constantly and with a uniform in-plane distribution.
On the other hand, an oxide film comprising ZnO or SnO2 as the main component is preferably used, for example, as a heat ray cut-off film.
As a film construction of a heat ray cut-off film to be used for buildings or automobiles, 1) a film having three layers of an oxide film, a Ag film and an oxide film sequentially laminated from the substrate side, or 2) a film having five layers of an oxide film, a Ag film, an oxide film, a Ag film and an oxide film sequentially laminated from the substrate side, is known. Such a heat ray cut-off film is called a Low-E (Low-Emissivity) film, and a glass having this film formed thereon is called a Low-E glass.
As a main Low-E glass, the one having a film construction of ZnO, Ag and ZnO sequentially laminated on a glass substrate, is available, and is usually used as a double glazing or a laminated glass. However, this film construction has a problem with respect to moisture resistance, and there is a problem in the storage stability or handling efficiency, since white spots are likely to form due to moisture in the environment during the storage of single plates after film formation before the step for double glazing or lamination.
As described in JP-A-4-357025, it is known that in order to improve the moisture resistance of Low-E glass, reduction of the internal stress of the uppermost oxide layer is effective. As an oxide film with a low internal stress, 1) a tin oxide film, or 2) a ZnO film having other element having an ion radius smaller than Zn2+ in an oxidized state (such as Al, Si, B, Sn, Mg or Cr) incorporated, is, for example, disclosed.
In a case where a laminate employing the above SnO2 film or the ZnO film, is to be formed by a large size film forming apparatus to be used for production of glasses for buildings or automobiles, even if the above-mentioned oxide film is selected for use, the internal stress of the oxide film can not sufficiently be reduced, since the amount of water remaining in the sputtering apparatus is substantial. As a result, the moisture resistance of the obtained laminate has been often inadequate, and an improvement has been desired.
It is an object of the present invention to provide (1) an oxide film having an internal stress lower than heretofore and a method for its production, (2) a laminate employing such an oxide film so that the moisture resistance is further improved, and the storage and handling efficiency in the form of single plates are further facilitated, and a method for its production, and (3) a method for producing an oxide film, whereby the desired electrical properties and optical properties can be constantly obtained in a good in-plane distribution.
The present invention provides a method for forming an oxide film on a substrate by a sputtering process using a target comprising a metal as the main component, wherein sputtering is carried out in an atmosphere which contains a gas containing carbon atoms.